The present invention relates to an active matrix type liquid crystal device including active elements located at respective pixels (also called picture elements) thereby controlling the voltage applied to the liquid crystal at the respective pixels, and more particularly, to a liquid crystal device of the type in which a voltage is applied in a lateral direction (along the layer) to the liquid crystal at the respective pixels.
The present invention also relates to electronic equipment using such a liquid crystal device.
The great majority of liquid crystal devices used in notebook personal computers or liquid crystal TV sets are operated in the twisted nematic mode. However, in the twisted nematic mode, the image displayed on a liquid crystal device looks different depending on the viewing direction. To improve the viewing direction dependence, it has been proposed to employ an in-plane switching (IPS) mode in which a voltage is applied to a liquid crystal in a lateral direction, as disclosed for example in Japanese Unexamined Patent Publication Nos. 56-091277 and 6-160878.
The principle of operation in the IPS mode will be described briefly with reference to some drawings. FIGS. 4a and 4b are cross-sectional views illustrating the behavior of a liquid crystal in a liquid crystal panel designed to operate in the IPS mode, wherein FIG. 4a is a cross-sectional view of a cell without an application of voltage and FIG. 4b is a cross-sectional view of the cell under the application of a voltage greater than a threshold value. The plane views of FIGS. 4a and 4b are given in FIGS. 4c and 4d, respectively. In FIG. 4, reference numerals 401 and 409 denote a pair of polarizing plates, 402 and 408 denote a pair of substrates between which a liquid crystal is disposed, 403 denotes a color filter, 404 and 406 denote orientating films, and 405 denotes a liquid crystal molecule drawn in a schematic fashion. Furthermore, reference numeral 410 denotes a pixel electrode, 411 denotes a common electrode disposed in a pixel at a location opposite to the pixel electrode, 412 denotes an image signal line (source line), and 407 denotes an insulating layer for isolating the pixel electrode 410 and the common electrode 411 from each other. In the IPS-mode liquid crystal device, as can be seen from FIG. 4, the pixel electrode and the common electrode for applying a voltage across the liquid crystal are disposed on one substrate at locations apart in a lateral direction. Reference numeral 413 denotes the absorption axis of the lower polarizing plate and 414 denotes the absorption axis of the upper polarizing plate.
Although an active element such as a TFT (thin film transistor) is also disposed, it is not shown in FIG. 4. FIGS. 4a and 4b are a cross section taken along line X-Xxe2x80x2 of FIG. 5, and FIGS. 4c and 4d are an enlarged plane view illustrating an area surrounded by a broken line in FIG. 5, wherein FIG. 5 illustrates the structure of one pixel. In the specific example shown in FIG. 5, two common electrodes 502 and one pixel electrode 501 are disposed in a lateral direction in one pixel, whereas there may be some other number of common electrodes 502 and pixel electrodes 501 in one pixel. Furthermore, in FIG. 5, reference numeral 503 denotes a scanning signal line (gate line), 504 denotes an image signal line (source line), and 505 denotes a thin film transistor(TFT).
Of the pair of substrates 402 and 408, as shown in FIGS. 4a and 4c, a color filter 403 is formed on the upper substrate 402, and a line-shaped common electrode 411 and pixel electrode 410 are formed on the inner surface of the lower substrate 408. Furthermore, orientating films 404 and 406 for orientating the liquid crystal molecules 405 are formed on the inner surfaces of the respective substrates. A liquid crystal is disposed between the pair of substrates 402 and 408. When no voltage is applied, the liquid crystal molecules 405 are uniformly orientated at a fixed angle (within the range from 0 to 45) with respect to the longitudinal direction of the line-shaped electrodes (common electrode 411, pixel electrode 410). In the specific example shown in FIG. 4, the angle is set to 30xc2x0. On both sides of the liquid cell, there are disposed polarizing plates 401 and 409. The upper polarizing plate 401 is disposed such that its absorption axis 414 becomes parallel to the orientation of the liquid crystal. On the other hand, the lower polarizing plate 409 is disposed such that its absorption axis 414 becomes perpendicular to the orientation of the liquid crystal. In this state, black is displayed in the pixel. The liquid crystal is made up of a material having positive dielectric anisotropy.
If an electric field 415 is applied, the liquid crystal molecules 405 are aligned so that their longitudinal axis is directed in a direction parallel to the electric field 415, as shown in FIGS. 4b and 4d. As a result, the orientation of the liquid crystal molecules 405 come to have a certain angle with respect to the absorption axis of the polarizing plates. The birefringence of the liquid crystal varies in accordance with the orientation angle of liquid crystal molecules which varies in response to the strength of the applied electric field. Thus, it is possible to control the transmission of light through the pair of polarizing plates thereby controlling the brightness.
In this structure, however, the pixel electrode 410 and the common electrode 411 used to apply a voltage across the liquid crystal are formed on only one substrate and there is no electrode on the other-side substrate. This can cause a problem in that the substrate tends to be electrostatically charged. The electrostatic charge disturbs the orientation of the liquid crystal and thus it becomes impossible to display a high-quality image. Once the substrate is electrostatically charged, it is difficult to remove the electrostatic charge because there is no electrode on the other-side substrate.
In view of the above, it is an object of the present invention to provide a liquid crystal device capable of displaying a high-quality image without being electrostatically charged or without being influenced by an electrostatic charge.
According to an aspect of the present invention, there is provided a liquid crystal device including a pair of substrates with a liquid crystal disposed between them, one of the substrates having, thereon, scanning signal lines and image signal lines disposed in a matrix form, active elements connected to the respective scanning signal lines and image signal lines, a pixel electrode connected to the respective active elements, and a common electrode, thereby making it possible to apply an electric field across the liquid crystal disposed between each pixel electrode and the common electrode in such a manner that the electric field is applied in a direction substantially parallel to the plane of the substrates, wherein a light-shielding metal film is formed on the other-side substrate opposite to the one of the substrate, and a fixed voltage is applied to the light-shielding metal film.
In this structure, the other-side substrate is prevented from being electrostatically charged and thus it is possible to display a high-quality image. If there were no light-shielding metal film disposed on the other-side substrate and maintained at the fixed voltage, the substrate would be electrostatically charged, and as high a voltage as a few ten thousand volts would occur between the other-side substrate and the pixel electrodes and/or the common electrode formed on the one substrate. The liquid crystal would response to that voltage. For the above reason, to achieve a high-quality image, it is important that the light-shielding metal film be formed on the other-side substrate having no electrode for driving. the liquid crystal and be maintained at a fixed voltage Preferably, the light-shielding metal film is made up of chromium (Cr) or a nickel-copper (Nixe2x80x94Cu) alloy.
According to another aspect of the present invention, there is provided a liquid crystal device including a pair of substrates with a liquid crystal disposed between them, one of the substrates having, thereon, scanning signal lines and image signal lines disposed in a matrix form, active elements connected to the respective scanning signal lines and image signal lines, a pixel electrode connected to the respective active elements, and a common electrode, thereby making it possible to apply an electric field across the liquid crystal disposed between each pixel electrode and the common electrode in such a manner that the electric field is applied in a direction substantially parallel to the plane of the substrates, wherein a transparent conducting film is formed on the other-side substrate opposite to the one of the substrates, and a fixed voltage is applied to the transparent conducting film.
With this structure, the other-side substrate is prevented from being electrostatically charged and thus it is possible to display a high-quality image. If there were no transparent conducting film disposed on the other-side substrate and maintained at the fixed voltage, the substrate would be electrostatically charged, and as high a voltage as a few ten thousand volts would occur between the other-side substrate and the pixel electrodes and/or the common electrode formed on the one substrate. The liquid crystal would response to that voltage. For the above reason, to achieve a high-quality image, it is important that the transparent conducting film be formed on the other-side substrate having no electrode for driving the liquid crystal and be maintained at a fixed voltage. Preferably, the transparent conducting film may be made up of ITO or tin oxide (SnO2).
According to another aspect of the present invention, there is provided a liquid crystal device including a pair of substrates with a liquid crystal disposed between them, one of the substrates having, thereon, scanning signal lines and image signal lines disposed in a matrix form, active elements connected to the respective scanning signal lines and image signal lines, a pixel electrode connected to the respective active elements, and a common electrode, thereby making it possible to apply an electric field across the liquid crystal disposed between each pixel electrode and the common electrode in such a manner that the electric field is applied in a direction substantially parallel to the plane of the substrates, wherein a conductive film is formed on the other-side substrate opposite to the one of the substrates, in the periphery of the pixel area on either the inner surface or the outer surface of the other-side substrate, and a fixed voltage is applied to the conductive film.
With this structure, the other-side substrate is prevented from being electrostatically charged and thus it is possible to display a high-quality image. If there were no conductive film disposed on the other-side substrate and maintained at the fixed voltage, the substrate would be electrostatically charged, and as high a voltage as a few ten thousand volts would occur between the other-side substrate and the pixel electrodes and/or the common electrode formed on the one substrate. The liquid crystal would response to that voltage. For the above reason, to achieve a high-quality image, it is important that the light-shielding metal film be formed on the other-side substrate having no electrode for driving the liquid crystal and be maintained at a fixed voltage. Since the conductive film is formed in areas outside the displaying areas, it is not required that the conductive film be transparent, and thus various metallic materials may be employed to form the conductive film.
Herein, the pixel area refers to such an area in which an element of an image such as a character or a picture is displayed. An example of a pixel area is an area 303 of the liquid crystal cell denoted by a broken line in FIG. 3. The periphery of the pixel area refers to such an area which is located outside the pixel area and which has no capability of displaying an image. In FIG. 3, reference numeral 304 denotes the periphery of the pixel area.
Preferably, the fixed voltage is either a ground voltage, a voltage on the common electrode, the center voltage of the image signal amplitude, a non-selection voltage of the scanning signal, or a logic voltage provided from an external driving means.
In this technique, a proper one of existing voltages in the liquid crystal device can be used without having to generate an additional voltage, and thus it is possible to realize a liquid crystal device capable of displaying a high-quality image and having high resistance to electrostatic charge without causing an increase in cost.
Herein, the common electrode voltage, the center voltage of the image signal amplitude, and the non-selection voltage of the scanning signal refer to such voltages denoted by reference numerals 606, 605, and 607, respectively, in FIG. 6 representing the waveforms of driving signals associated with a liquid crystal panel with TFTs. The waveforms of the driving signals shown in FIG. 6 are described briefly below in junction with an equivalent circuit of a TFT shown in FIG. 7. Signals 602 and 603 are supplied over a scanning line 703 and an imaging signal line 704, respectively, and applied to the gate and source, respectively, of the TFT 705. According to the NTSC standard, an image signal consists of two interlaced fields, that is, a first field 610 and a second field 611, which make up one frame 612 thereby making up one picture. In a selection period 608, if the TFT 705 is turned on by a selection signal supplied via the scanning signal line 703, the voltage 604 of the pixel electrode 701 becomes nearly equal to the voltage 603 of the image signal line 704. In a non-selection signal 609, the TFT 705 is turned off and the signal written in a liquid crystal capacitor 706 is held. The scanning signal lines 703 are selected one by one in a similar manner so that data is rewritten once every one field for all pixels.
According to another aspect of the present invention, there is provided a liquid crystal device including a pair of substrates with a liquid crystal disposed between them, one of the substrates having, thereon, scanning signal lines and image signal lines disposed in a matrix form, active elements connected to the respective scanning signal lines and image signal lines, a pixel electrode connected to the respective active elements, and a common electrode, thereby making it possible to apply an electric field across the liquid crystal disposed between each pixel electrode and the common electrode in such a manner that the electric field is applied in a direction substantially parallel to the plane of the substrates, wherein a polarizing plate having an electrical conductivity is disposed on the outer surface of the other-side substrate opposite to the one substrate, and a fixed voltage is applied to the polarizing plate.
In this structure, the other-side substrate is prevented from being electrostatically charged and thus it is possible to display a high-quality image. If there were no conductive film disposed on the other-side substrate and maintained at the fixed voltage, the substrate would be electrostatically charged, and as high a voltage as a few ten thousand volts would occur between the other-side substrate and the pixel electrodes and/or the common electrode formed on the one substrate. The liquid crystal would response to that voltage. For the above reason, to achieve a high-quality image, it is important that the polarizing plate having an electrical conductivity be formed on the other-side substrate having no electrode for driving the liquid crystal and be maintained at a fixed voltage.
In the present invention, the above-described fixed voltage is either a ground voltage, a voltage on the common electrode, the center voltage of the image signal amplitude, a non-selection scanning signal voltage, or a logic voltage provided from an external driving means.
According to another aspect of the present invention, there is provided a liquid crystal device including a pair of substrates with a liquid crystal disposed between them, one of the substrates having, thereon, scanning signal lines and image signal lines disposed in a matrix form, active elements connected to the respective scanning signal lines and image signal lines, a pixel electrode connected to the respective active elements, and a common electrode, thereby making it possible to apply an electric field across the liquid crystal disposed between each pixel electrode and the common electrode in such a manner that the electric field is applied in a direction substantially parallel to the plane of the substrates, wherein a transparent conducting film is formed on either the inner-side or outer surface of the other-side substrate opposite to the one substrate and the voltage of the transparent conducting film is maintained in a floating state.
In this structure, the other-side substrate is prevented from being electrostatically charged and thus it is possible to display a high-quality image. Even if the liquid crystal device is partially charged, the transparent conducting film prevents the orientation of the liquid crystal from being locally disturbed, and the charge is relaxed over the conductive film. Furthermore, in this technique, since the voltage of the transparent conducting film is maintained in a floating state, no electrical connection is required. In general, if the transparent conducting film is formed on the inner surface of the substrate, that is, in the inside of a liquid crystal cell, a voltage difference occurs between the transparent conducting film and the pixel electrode and/or the common electrode. As a result, degradation in the image quality occurs. However, in the present technique, since the voltage of the conductive film is maintained in the floating state, the degradation in the image quality is suppressed. Preferably, the transparent conducting film is made up of ITO or tin oxide (SnO2).
Herein, the floating state refers to such a state in which a conductive material is electrically isolated from any voltage in adjacent locations and thus the voltage of the conductive material is in a floating state.
According to another aspect of the present invention, there is provided a liquid crystal device including a pair of substrates with a liquid crystal disposed between them, one of the substrates having, thereon, scanning signal lines and image signal lines disposed in a matrix form, active elements connected to the respective scanning signal lines and image signal lines, a pixel electrode connected to the respective active elements, and a common electrode, thereby making it possible to apply an electric field across the liquid crystal disposed between each pixel electrode and the common electrode in such a manner that the electric field is applied in a direction substantially parallel to the plane of the substrates, wherein a conductive film is formed on the other-side substrate opposite to the one of the substrate, in the periphery of the pixel area on either the inner surface or the outer surface of the other-side substrate, and the voltage of the conductive film is maintained in a floating state.
In this structure, the other-side substrate is prevented from being electrostatically charged and thus it is possible to display a high-quality image. Even if the liquid crystal device is partially charged, the transparent conducting film prevents the orientation of the liquid crystal from being locally disturbed, and the charge is relaxed over the conductive film. Furthermore, since the voltage of the transparent conducting film is maintained in a floating state, no electrical connection is required. Since the conductive film is formed in areas outside the displaying areas, it is not required that the conductive film be transparent, and thus various metallic materials may be employed to form the conductive film.
According to another aspect of the present invention, there is provided a liquid crystal device including a pair of substrates with a liquid crystal disposed between them, one of the substrates having, thereon, scanning signal lines and image signal lines disposed in a matrix form, active elements connected to the respective scanning signal lines and image signal lines, a pixel electrode connected to the respective active elements, and a common electrode, thereby making it possible to apply an electric field across the liquid crystal disposed between each pixel electrode and the common electrode in such a manner that the electric field is applied in a direction substantially parallel to the plane of the substrates, wherein a polarizing plate having an electrical conductivity is disposed on the outer surface of the other-side substrate opposite to the one substrate, and the voltage of the polarizing plate is maintained in a floating state.
In this structure, the other-side substrate is prevented from being electrostatically charged and thus it is possible to display a high-quality image. Even if the liquid crystal device is partially charged, the transparent conducting film prevents the orientation of the liquid crystal from being locally disturbed, and the charge is relaxed over the conductive film. Furthermore, since the voltage of the transparent conducting film is maintained in a floating state, no electrical connection is required.
In this structure according to the present invention, the conductive film is formed on the inner or outer surface of the other-side substrate thereby ensuring that the orientation of the liquid crystal is maintained in a desired direction without encountering disturbance. Although the external electrostatic charge can be absorbed even if the conductive film is formed on either the inner or outer surface of the other-side substrate, it is more preferable that the conductive film be formed on the inner surface of the other-side substrate so that the electrostatic charge is absorbed at a location nearer to the liquid crystal layer.
The liquid crystal device constructed in any form described above may be employed as a display device in various electronic equipment.